1. Field of the Invention
This invention relates to check valves, and in particular, to an improved shaft seal for use on a check valve.
2. Description of the Prior Art
It is well known in the nuclear steam power generation art that water used in the closed turbine loop of a nuclear steam power plant must be chemically treated through the addition of certain additives. Such additives, including phosphates, are required in order to provide protection for the materials utilized to fabricate the various elements comprising the closed turbine loop. Such protection is especially required in order to protect the material used to fabricate the steam generator element from deleterious effects caused by passage therethrough of overly caustic secondary water.
Of particular benefit is phosphate, which, when added to the secondary water, combines with free hydrogen molecules carried therein to prevent the secondary water from becoming overly caustic and thus to prevent damage to the metals used in fabricating various elements of the closed turbine loop. However, as is well known to those skilled in the art, phosphate is soluble in water. Thus, phosphate is carried by the moisture in the steam throughout the entire system. The presence of phosphate in solution is troublesome in valving, and is especially troublesome within the turbine main stop valve.
The turbine main stop valve is usually a clapper or a check type valve which is utilized in series with a plug-throttle valve. Both the check valve and the plug valve are usually interposed between the steam generator and the turbine elements of the closed turbine loop. The plug-throttle valve is a non-zero leakage valve which indicates that some leakage flow can occur therethrough. Thus the primary cut-off function devolves upon the check or clapper valve. If this valve fails, the possibility of damage to the turbine element due to occurrence of an overspeed condition increases.
The present clapper valve has a valve shaft rotatably disposed within a valve housing. The valve shaft supports a valve disc which moves from an open to a seated position within a valve chamber defined by the interior of the housing. A bushing member is tightly fitted about the shaft between the housing and the shaft. A leakoff gland is disposed within the housing. The leakoff gland is connected to a condenser element and is typically at a pressure lower than that of the atmosphere.
Presently, sealing means are disposed between the shaft and the bushing at a point on the valve shaft adjacent the bushing yet still within the valve chamber. With the valve disc in the open position, the seal is fully engaged, and thus a flow of steam from the valve chamber to the gland condenser is to be prevented by the shaft sealing means.
However, the shaft seal as presently employed at time does not seal effectively. As a result, a flow of steam passes from the valve chamber, through the close clearance between the valve shaft and the bushing, to the leakoff. It is well known to those skilled in the thermodynamic art, that the throttling of wet steam from a first pressure to a lower pressure in the presence of a constant temperature will drive the steam into the superheat region. It is also well known that steam in the superheat region is unable to hold materials carried in solution thereby. Since the steam which leaks past the present shaft seal in check valves contains dissolved foreign matter, especially phosphate, in solution, the throttling of such leakage steam at the present seal interfaces results in the deposition of phosphate precipitate adjacent to the location of the shaft and the present bushing.
Such deposition of phosphate precipitate accumulates and eventually causes the narrow clearances between the shaft and the bushing to close. It is apparent that such accumulation of phosphate precipitates increases the amount of torque required to close the valve. In practice, it has been observed that the deposits are sufficient to cause the valve to fail to operate entirely. Such accumulation of phosphate precipitates are usually insidious, in that the effect of their accumulation is not known or discernible by any method other than an attempt to close the valve.
The buildup of such deposits can be eliminated in a variety of ways. Provision of a better shaft seal, for example, so that a leakage flow does not occur will solve the problem. However, provision of a perfect sealing device between a rotatable shaft and a shaft support is very difficult.
The problem may also be overcome by eliminating the moisture content in the steam supply or eliminating the presence of foreign matter within the secondary water. Since total elimination of the moisture content in the steam supply is also an untenable solution, the remaining alternative is to eliminate the presence of foreign matter from the secondary water. But as mentioned above, this solution is impractical since it would also eliminate the method by which the metals used to fabricate various elements in the closed turbine loop are protected from caustic agents.